This invention relates to heating a plasma by means of anomalous absorption of a laser beam, and more particularly it relates to the control of plasma density perturbations to enhance the absorption of the laser beam.
The transfer of electromagnetic energy from a single laser beam to a target plasma is initiated by random ion density fluctuations that are naturally present in a plasma. Under the action of the electromagnetic wave field of the laser beam, the plasma electrons oscillate at the laser frequency within the plasma back and forth past the pre-existing natural ion density fluctuations. The period of this electron oscillation is short as compared with the characteristic time for substantial change in the ion density pattern. Together with the natural ion density fluctuations there exist comparable electron density fluctuations, the electrons tending to follow the ion density fluctuations. Concomitant with the electron oscillations, electron density waves known as electron Langmuir waves (and certain other types of density variations) are directly generated. Electron Langmuir waves are undulations of the particle density of electrons in the plasma, which pattern of electron density moves through the plasma with a characteristic phase velocity depending on the wavelength of the undulations and the electron temperature, and which is accompanied by a longitudinal electric wavefield whose frequency is of the order of the electron plasma frequency ##EQU1## WHERE E, N.sub.E AND M.sub.E DENOTE RESPECTIVELY THE ELECTRON CHARGE, THE ELECTRON DENSITY AND THE ELECTRON REST MASS.
Although the initial ion density fluctuations may be relatively small compared with the average ion density, there exists electrostatic coupling between the ion density fluctuations and the driven electron density waves which causes immediate growth, called parametric instability, in the amplitude of the ion density fluctuations as well as the associated electron Langmuir waves. During this growth in amplitude, energy transfer occurs from the laser beam to these plasma density waves. When the density perturbations associated with these waves become comparable with the averge ion density, the energy associated with the waves at that time is converted by turbulent processes into thermal energy in the plasma. The corresponding attenuation of the laser beam is known as anomalous absorption (as opposed to collisional absorption).
A persistent problem in heating a plasma by anomalous absorption is the generation of electron Langmuir waves having substantially larger phase velocities than the electron thermal velocity. The electric field of such Langmuir waves does not readily couple to plasma electrons which are near the thermal velocity and for this reason they are ineffectual in heating the plasma. Instead, the high-velocity waves act randomly to accelerate some of the electrons to superthermal velocities. This problem arises because the wave number spectrum of the typical ion density fluctuations extends downward to such small wavenumbers that the corresponding phase velocities of the electron Langmuir waves may considerably exceed the electron thermal velocity.